The present invention relates to an open-controlled serial printer and, more particularly, to detection of offsetting of the carriage from its intended position.
A conventional printer of this kind usually has the construction comprising, as shown in FIG. 1, a platen 1, a carriage 2 for setting the printing position in the line direction, a belt 3 for driving the carriage 2, a daisy wheel 5 having types 5a, 5b, 5c . . . formed at ends of respective spokes, a daisy wheel motor 6 for driving the daisy wheel 5, and a hammer 7 applying a printing impact to the types.
A sensor device 11 comprising a light emitting element 9 and a light receiving element 10 is secured to the carriage 2. The sensor device 11 is constructed so as to move with the carriage 2 as one body. The printer of FIG. 1 further comprises a sensor bar 12 secured to the position at which the left margin is zero and is so constructed that the light path of the sensor device 11 is obstructed by the sensor bar 12 when the carriage 2 takes the position at which the left margin is zero or the home position. In FIG. 1, the sensor bar 12 is shown at a position difference from the position at which it should be in actual construction.
In the printer of the construction shown in FIG. 1, the carriage 2 is driven by the carriage motor to determine the printing position in the line direction, the daisy wheel 5 is rotated by the daisy wheel motor 6 to select the printing type, and, thereafter, the printing type at the end of the spoke is given an impact by the hammer 7 from the reverse side toward the platen 1 to print on a printing paper 13.
Heretofore, in the printer of this type, in order to detect whether the accurate printing position, that is the position of the carriage 2, is selected or not (hereinafter called out of step sensing), the printer is so adjusted that the carriage 2 is returned to the home position upon initializing the printer and the sensor bar 12 obstructs the light path of the sensor device 11 (dark check). After the printer has been initialized, if the left margin is set to zero, the dark check is performed to detect carriage offset when the carriage return order is executed or each time form feed (F.F. operation) is carried out.
However, the conventional printer of the construction described above has a disadvantage that accurate out of step sensing of unintended carriage offset is impossible in the case where the carriage 2 is stepped out or offset toward left.
This disadvantage will be described with reference to FIGS. 1 and 2 in which the same component parts are denoted by the same reference numerals. The sensor bar 12 is secured in the proximity to a frame 15 to minimize dead zone. If dark check is carried out in the state in which the carriage is out of step by X.sub.0 toward right, a carriage motor is applied with a number of steps corresponding to a theoretical distance .alpha. required to return the carriage 2 from the print position, according to the total number of steps of the motor, to a home position P.sub.1. As the result, the carriage 2, that is the sensor device 11, returns to a point R which is spaced from the home position P.sub.1 to right by X.sub.0. If dark check is carried out in this state, the light path of the sensor device 11 is not obstructed by the sensor bar 12 and applies a light signal, whereby the out of step is detected.
On the other hand, if the dark check is carried out in the state in which the carriage 2 is out of step or offset toward left by X.sub.0, the carriage motor is applied with a number of steps corresponding to the distance .alpha. to thereby, theoretically, return the carriage 2, that is the sensor device 11, to a point L which is spaced from the home positon P.sub.1 to left by X.sub.0.
In fact, however, in the case where the carriage 2 is out of step or offset toward left, the carriage 2 is brought in contact with the frame 15 and, accordingly, the carriage 2, that is the sensor device 11, stops at a position M at which the carriage 2 is in contact with the frame 15. In this state, therefore, despite the offset of the carriage 2, a dark signal is applied from the sensor device 11, to thereby decide that there is no offset. As described above, the conventional printer of this type has a disadvantage that accurate out of step detection is impossible with it.